1. Field of the Invention
This invention relates to integrated satellite/cell telecommunication systems and more particularly to systems and methods for providing low-cost telephony services to remote rural and other previously non-served subscribers by means of distributed switching networks.
2. Description of the Related Art
There is an increasing demand for telephone (voice, fax and data) communication capability. There are three basic forms of telephone service transmission media: guided, unguided and semi-guided. Guided basically entails "hard wired" systems such as wire cable, fiber optic cable, coaxial cable and the like. These systems, while very secure and durable, are capital intensive to install and maintain. Terrain and distances make these systems economically prohibitive.
Unguided media connect through signals propagated into free space. Satellites and wireless telephone systems use this media. As traffic increases the bandwidth required for this type of telephone communication becomes prohibitive. Finally, semi-guided media, such as microwave, links channeled signals from point to point without physical constraints, but are limited to short distances. Thus, telephony services using this medium exclusively are distance limited.
There are many countries around the world that lack a guided telecommunications infrastructure. These are primarily emerging countries, which have remote or rural areas of low density population requiring subscriber telecommunications capability to provide quality communication services. In these emerging or developing countries, the existing infrastructure is insufficient to satisfy the individual subscriber telecommunications requirement of a large portion of the country, and especially the needs of subscribers in remote/rural towns and villages.
Wireless communication for voice, fax, and data services is the most rapidly developing sector of telecommunications. This is especially true in sparsely and rurally populated developing countries where guided systems are prohibitive. Distributed wireless networks have been developed to connect cells of subscribers and/or a cell to a public switched telephone network. A telephone architecture wherein a portion of the interconnect between an individual subscriber unit and a public switched telephone network is accomplished by radio communication is called a wireless local loop.
Heretofore, there have been three basic techniques that can be used to provide subscriber telephony service: (1) cellular, (2) rural access time division multiple access and (3) satellite. These have been applied both separately and in various combinations (so called hybrid systems). Cellular, both analog and digital, is efficient in providing the subscriber inter-cell service using relatively low-cost subscriber equipment. The low cost effectiveness of this service, however, depends on the existence of a cost effective infrastructure to provide an interconnect to other cells as well as the public switched telephone network.
The rural access microwave and wireless time division multiple access systems work well for intra cell connections of cell subscribers as well as the public switched telephone network if distances are short i.e. less than 100 km. But the capital equipment costs for these systems are very high. Finally, satellite systems provide direct connection of all subscribers to other subscribers, as well as the public switched telephone network, usually by means of very small aperture terminals which are distance insensitive, but like time division multiple access, are relatively expensive for the subscriber. As traffic increases bandwidth demands become excessive.
Hybrid systems offer the possibility of providing low-cost subscriber equipment and low cost connection to the public switched telephone network. Thus, theoretically, cellular could be combined with satellite to provide low-cost subscriber equipment with an efficient interconnect to other cells of distances more than 100 km and the public switched telephone network.
Typical cellular systems have one large switching system separating the cell from the public switched telephone network. This large switching system performs a switch/network control function, and is referred to as a mobile switching center or a mobile telephone switching office. The centralized switch maintains a register of all subscriber locations to effect routing both inter-cell and intra-cell. The location register for a particular cell is a database containing the names, ID's, etc. for all subscribers within that cell. In addition, for cellular systems where one may roam into adjacent networks, a visitor location register is maintained that contains the names, ID's, etc. of visitors to the cellular network. (Visitor location registers are not necessary in fixed based wireless local loops.)
In a traditional hybrid cellular/satellite network, all traffic is routed from a base transceiver system through the satellite to the mobile switching center for both the connection to the public switched telephone network and for intra-cell/inter-cell switching functions. In this architecture, an intra-cell and inter-cell call actually requires a double-hop or two communication traffic circuits between the mobile switching center and the remote cell. Since the greatest drawback to satellite communications for telephony applications is the delay introduced when using geosynchronous satellites, double hop systems will exacerbate this delay, use excessive bandwidth and generally degrade network operations.
Full mesh, single hop systems, on the other hand, constrain the delay to a single hop for good voice performance. In a full mesh system, only a single hop is required for inter-cell calls. If operated in a demand assigned multiple access (DAMA) mode, the amount of satellite bandwidth required for cost effective network operations is greatly reduced. In demand assigned mode, all calls are assigned only when satellite bandwidth is required. A request is made (from a cellsite or gateway) for access to the satellite only when a circuit is required to complete a given voice/fax/data call. This technique greatly reduces the amount of bandwidth required since bandwidth can be treated as a pool of channels, shared by the entire network. If channels are not assigned on-demand, then trunks must be maintained between every cellsite and gateway in the network in order to support a single hop operation. This is an exponential function: one trunk between two sites, three trunks between 3 sites, 6 trunks between 4 sites and so forth. As the number of interconnect sites becomes larger, the amount of bandwidth required becomes unsupportable. Additionally, the number of channels supported at each site grows to where it is cost prohibitive. With state of the art voice compression algorithms and demand assigned techniques, the satellite bandwidth required is minimized reducing network operational costs. Small aperture terminals or very small aperture terminals are used to provide the link between the cell base station and another satellite earth terminal.
It would therefore be desirable to have a system designed to avoid the back haul interconnects i.e. double hops to this large switching system or mobile switching center. If a distributed network could be provided where cellular or cordless phone technology was used at the "cellsite" to provide the lowest cost subscriber equipment and a satellite infrastructure could be used to enable the cell to be placed virtually anywhere, the satellite system could be operated as a "demand assigned multiple access" (DAMA) system communicating traffic with "full mesh" capability, i.e. any subscriber being able to communicate with any subscriber or with the public switched telephone network while requiring only a single "satellite hop" (single full duplex channel). If an intra cell call could be switched locally, it would not require access to satellite bandwidth. The result is a system that minimizes the subscriber equipment cost, minimizes network operational cost, and provides the ability to build and connect cells virtually anywhere while eliminating the need for large switching centers to handle data communications.
Various proposals in the prior art have attempted to solve this rural telephony dilemma in a number of ways, all with only a modicum of success. For example, U.S. Pat. No. 5,280,472 teaches a microcellular telephone system that utilizes satellites in combination with a wireless local loop. A wireless local loop (wireless PBX) system uses a distributed antenna system consisting of a set of simple antennas fed by common signal in order to cover a given area. Time delays are introduced between antennas so that signals received from two antennas are distinguishable.
U.S. Pat. No. 5,742,640 discloses a system which uses wireless access between subscribers and the base station. The base station is connected to the controller by means of a wire line encrypted modem link. The controller provides a physical connection to public switched telephone network. All calls are, by definition, routed to the public switched telephone network for any switching. This system is not appropriate for use with a satellite link between the cell and the public switched telephone network.
U.S. Pat. Nos. 5,748,610, 5,663,990 and 5,614,914 disclose satellite links in combination with a public switched telephone network system for distributing calls.
U.S. Pat. No. 5,708,679 teaches a satellite system that transmits to multiple remote terminals.